1. Field of the Invention
The present invention relates to a hydraulic circuit for controlling a continuously variable ratio unit (“variator”) of the toroidal-race rolling-traction type, and more particularly for controlling an end load in such a variator.
2. Background Art
Toroidal-race rolling-traction type variators are in themselves well known. One, or more typically two, toroidal or part toroidal cavities are defined by opposed faces of rotatably, coaxially mounted discs and drive is transmitted between the discs by rollers disposed in the cavities. It is well known in such variators to mount each roller in a carriage and to connect that carriage to a piston subject to a controlled hydraulic force. So-called “torque control” operation can be achieved in well known manner by applying the hydraulic force along a generally tangential direction (with respect to the axis of the variator discs) and allowing the roller/carriage to move along a circular path centred on the axis. The roller is permitted to precess (that is, the roller axis can rotate) and as is well known the roller precesses such that its axis always intersects the disc axis. Consequently when the roller moves along its circular path it also precesses and the change in roller inclination produces a change in variator transmission ratio. The roller adopts a position in which the force applied thereto by the piston is balanced by an opposite, “reaction”, force produced (by shear of a film of so-called “traction fluid”) at the interfaces between the roller and its neighbouring discs. The torque transmitted by the variator is a function of the reaction force. In the steady state the hydraulic and reaction forces balance.
In order to enable transmission of torque by the variator there must be pressure at the roller/disc interfaces and in variators of the “full toroidal” type this is typically provided by means of a hydraulic actuator which acts on one of the variator discs to apply an “end load” biasing the discs toward the rollers. The magnitude of the end load has an important bearing on variator efficiency and performance. It is known to vary the end load during operation. An important parameter in this regard is the traction coefficient. If we define the normal force to be the force exerted by the roller on one of the discs (and of course by the disc on the roller) at the interface therebetween and along the direction normal to this interface, then the traction coefficient μ is simply the ratio of reaction force (RF) to normal force (NF):
  μ  =      RF    NF  
Note that the normal force is in the general case not precisely equal to the end load because the normal force acts along a direction perpendicular to the plane of the roller/disc interface, this direction being parallel to the direction of action of the end load only in one particular roller position (corresponding to a 1:1 variator drive ratio). In the general case, the end load and normal force are related through the cosine of the roller angle.
An excessively low traction coefficient, corresponding to an unnecessarily high end load and hence high normal force, gives rise to large energy losses at the roller/disc interface and so is inefficient. An excessively high traction coefficient is also inefficient in energy terms and can lead to variator failure, excessive slip at the roller/disc interface allowing the roller to move, rapidly in some situations, away from its proper position. It is necessary to guard against this eventuality.
In so called “full toroidal” variators, energy losses at the roller/disc interface can be considered in terms of (1) slip and (2) spin. Slip involves relative motion, along the circumferential direction, of the roller/disc surfaces at their interface, corresponding to a mismatch in rotational speeds of the roller and disc. Slip losses increase as the degree of slip increases. Spin involves relative angular motion of the two surfaces at the roller/disc interface. It arises from the geometry of the variator and the degree of spin is determined by this geometry, the roller positions and the variator speed. However energy losses due to spin are affected by the magnitude of the normal force and hence are related to the traction coefficient. It is found that the curve representing variation of efficiency with traction coefficient has a peak representing the best compromise between spin and slip losses. This must be taken into account in order to operate the variator at optimal efficiency.
A known hydraulic circuit for controlling the variator uses a pair of hydraulic lines to supply hydraulic fluid at adjustable pressures to opposite sides of the roller control pistons, thus enabling the reaction force to be varied. In order to provide for adjustment of end load, a valve arrangement of “higher pressure wins” type is used to supply fluid from whichever of the lines is at higher pressure to a working chamber of a hydraulic end load actuator and in this way a relationship is created between the reaction force and the normal force (or, to be strictly accurate in view of the cosine variation of normal force with roller angle referred to above, between the reaction force and the end load). One such arrangement is described in the applicant's earlier European Patent EPO894210 and in its US counterpart U.S. Pat. No. 6,030,310 which disclosed in detail a practical end loading arrangement and the contents of which are incorporated herein by reference for purposes of US law. In that arrangement the end load actuator actually has two working chambers, one supplied with pressure from the higher pressure line to apply the end load and one supplied from the lower pressure line to produce an opposed force which reduces the end load. In such an arrangement the traction coefficient can in effect be pre-set by appropriate choice of piston areas, particularly in the end load actuator.
The hydraulic coupling of the end load to the roller control actuators makes it possible to vary the end load rapidly in sympathy with the reaction force. This hydraulic coupling is highly advantageous because variators in motor vehicle transmissions are subject in practice to rapid and severe “torque spikes”, eg. upon braking, and to provide adequate end load on demand to accommodate such spikes (and avoid variator failure due to the traction coefficient increasing excessively) requires correspondingly rapid end load adjustment. In the arrangement described above, occurrence of a torque spike results in a corresponding pressure increase in the higher pressure line which is automatically and rapidly passed on to the end load actuator by the hydraulics.
However such systems are subject to problems. In some arrangements poor pressure response, in particular a time lag in matching the variator end load to the roller reaction force, has been found to occur. Unavoidably, compliance in the variator and its hydraulics mean that a finite volume of fluid is required to effect a change in end load. Flow is absorbed, eg. by flexure of the end load actuator components. In conjunction with flow restrictions in the hydraulics, the result can be a significant time lag between the reaction pressure and the end load pressure and hence a transient mismatch between the end load and reaction forces. The mismatch occurs during rapid changes in reaction force as in the event of transient torque spikes. In extreme cases there is an associated risk of variator failure.
It should be noted that EP0894210 suggests an arrangement in which a hydraulically controlled valve is used to control the end load pressure. This valve has a spool which is influenced by the end load pressure itself and also by mutually opposed pressures from opposite sides of the variator's double acting roller control pistons. The spool's position is determined by the balance between these three signals. The end load actuator is normally connected to a pump supplying pressurised fluid and the valve controls a drain from the end load actuator so that in response to excess end load pressure the drain is opened and the pressure is reduced. The arrangement is intended to maintain the traction coefficient at a constant level and there is no provision for adjustment of the traction coefficient.
It is desirable to provide for controlled adjustment of the traction coefficient, to make possible increased efficiency and to take account of variable factors such as the temperature of the variator traction fluid. Following start up the traction fluid, initially cold, is progressively warmed and its characteristics are consequently altered. The appropriate traction coefficient is likewise altered and it would be advantageous to carry out corresponding modification of the end load.
This need to adjust the traction coefficient according to temperature has been recognised in prior U.S. Pat. No. 6,162,144, assigned to General Motors Corporation, although the hydraulic circuit used to achieve the adjustment (see FIG. 3 of the patent) simply uses a pulse width modulated valve to feed a percentage of the end load pressure to a second chamber of the end load actuator, working in opposition to the end load pressure, to thereby adjustably reduce the end load. The additional problem of time lag in adjustment of the end load is not addressed. Additionally it is believed that there would be severe difficulties in creating a practical implementation of the circuit proposed in this patent, particularly in providing a pulse width modulated valve capable of carrying out the required function.
It should also be noted that adjustment of the coefficient of traction can be achieved in the type of known hydraulic circuit discussed above, having two hydraulic supply lines feeding opposite sides of the roller control pistons and a higher pressure wins arrangement to feed pressure from one of the lines to the end load actuator, by adjusting the pressures in both lines together to thereby increase or decrease the higher pressure (and hence the end load) without altering the pressure difference between the two lines which determines the reaction force. However this approach does not address the problem of end load time lag and complicates the control of the variator rollers.